Alluvial Sandstone Composition and Paleoclimate, I. Framework Mineralogy

ABSTRACT Systematic variations in compositional maturity of first-cycle fluvial sandstones are found in the Cutler Formation (Permian) and Fountain Formation (Permian-Pennsylvanian) in Colorado and in the Gondwana Supergroup (Permian-Triassic) of Peninsular India. These variations reflect changing climate during deposition. Climate is considered to be the critical factor affecting maturity because source rocks (granite, granite gneiss) and tectono-environmental setting (alluvial systems associated with basement-cored block uplifts) were similar and remained relatively unchanged throughout deposition of all three of the units. The roughly 3,200-m-thick Gondwana succession, consisting of five sandstone petrofacies, is characterized by the following sequence of change (oldest to youngest) in compositional maturity expressed in Q/F/R percentages: 54:42:4 88:10:2 62:34:4 82:16:2 99:1:0. This sequence is a function of the changing paleoclimate (glacial arid temperate humid warm semiarid warm semihumid warm humid) associated with the overall global limatic change and the changing latitudinal location of India as it moved from a location close to the South Pole northward during the interval of time represented by Gondwana deposition. Lithologic and fossil evidence indicates that, during deposition of the Fountain Formation, climate gradually changed from relatively wet and warm to at least semiarid and warm. This climatic change is the main factor responsible for the difference in compositional maturity of the older Fountain sandstone (Q:F:R-65:26:9) relative to the younger (Q:F:R-54:36:10). Cutler sandstone was deposited in an arid climate and has maximum compositional immaturity (Q:F:R-49:44:7). A log/log plot of the ratio of total quartz to total feldspar plus rock fragments against the ratio of total polycrystalline quartz to to al feldspar plus rock fragments is a sensitive discriminator of first-cycle sandstones with differing climatic heritage. Bulk chemical composition data support interpretation of climate from framework mineralogy, but by themselves are not sensitive enough to be unequivocal indicators. The same is true for the ZTR index and observations of degree and abundance of solution pits/embayments on detrital quartz. Results of this study support the conclusion of earlier empirical and theoretical analyses which suggest that the optimum conditions for the production and preservation of a distinctive climatic signature on sand composition are met in extensional plate-tectonic settings. Such settings are characterized by coarse-grained crystalline parent rocks, short transport distances in low-order streams, deposition in nonroutine environments, and shallow-burial diagenesis. However, even in such optimum settings, only rarely will climate be a more important determinant of sandstone composition than the tectonic setting itself.